Splines are a plurality of tooth-shaped keys provided on an outer circumferential surface of a shaft member, and are to be fitted into a member so that power (rotational force) can be transmitted through the mated members. In general, the splines are formed on an outer circumferential surface of a workpiece by a rolling operation using a rolling tool. Each of the splines can be easily broken at a longitudinal end portion of its root (at its ramped end portion). Therefore, there is a need for obtaining a torsional strength of such an easily breakable portion of the root as indicated by Japanese Application Laid-Open Publication No. JP-H11-290978 (see paragraph [0004], etc).
In view of the above need, the applicant of the present application invented a spline 100 as shown in FIG. 8 (which is not publicly known at the time of filing of the present application). As shown in FIG. 8, a shoulder 101a is provided in a ramped end portion of this spline 100, so that a root diameter defined by a root 101 of the spline 100 is changed in a direction of axis O (horizontal direction as seen in FIG. 8) of a shaft member. The provision of the shoulder 101a assures a strength of the ramped end portion, namely, makes it possible to improve the torsional strength.
In a rolling tool used for rolling the spline 100, one of widthwise end portions of each of its forming teeth is subjected to a crest removing operation (see FIG. 2), for permitting the shoulder 101a to be provided in the root 101 when the spline 100 is formed by the rolling tool.